Response of systemic arterial input impedance to volume expansion and hemorrhage

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Abstract
Experiments on 12 anesthetized dogs were performed to study the effects of changes in blood volume on the pulsatile hemodynamics of the arterial system as seen from its input. Pressure and flow were measured in the ascending aorta under control conditions, after volume expansion with dextran 70 (+30% of estimated blood volume) and after hemorrhage (-15% of estimated blood volume). The input impedance of the arterial system was calculated for each condition. After volume expansion the characteristic impedance of the proximal aorta, Zc, was decreased by 26.6 .+-. 5.1% (SE) (P < 0.01). After hemorrhage Zc was increased by 30.4 .+-. 3.4% (P < 0.01). Zc is a very weak function of the mean arterial pressure; the changes in Zc seen with volume expansion or hemorrhage apparently are caused mainly by changes in aortic smooth muscle activity. This conclusion is supported by direct measurements of aortic pressure diameter relationships in earlier work.